This week's vote on 42-day pre-charge detention is not about Gordon Brown, though over the past few days, moments of vanity, daring and panic have suggested it was. It is not about the Labour party or the future of the many decent people on the government backbenches. It is not about the opposition which hopes to stun the Prime Minister with a sensational defeat. It is not even about the government's 'independent' reviewer of terror legislation, Lord Carlile, who, for a Liberal Democrat lawyer, has the singularly weird mission of advocating ever greater departures from the constitution.

The proposal to lock up people for six weeks without trial, instead of four, is about nothing less than the quality of our democracy and the accumulated values of British culture. It is about our free society and the example we once hoped to set the world. It is about remembering that human rights are bought and maintained with hardship, risk and self-control.

To many voters, the issue may seem of minor academic interest in comparison to fuel and food prices and the collapse in the property market, but the truth is that we come to a moment of intense national significance which one way or the other will establish the direction and tone of our country. It will certainly determine whether this parliament continues its historic attack on rights and liberty or at last decides to reverse the trend of the Blair years.

But this is not an easy decision for Labour waverers, who find themselves torn between the principles of the Magna Carta, which they all understand perfectly well, and a tribal loyalty that is made more compelling by the disarray in Gordon Brown's premiership. It requires genuine courage for the people following the rebels' leader Frank Dobson to resist the appeals and threats of the party managers. We should not underestimate the great pressure they are experiencing.

Gordon Brown presents this as a stand of a strong Prime Minister who is making a tough and principled decision for the country's benefit, whereas in reality it is as much about his need to establish authority as anything else. It is a dreadful irony that his weakness may end up as one of the best allies of a project which, in the words of one Labour backbencher I talked to, is 'wrong, unnecessary and counterproductive'.

Yet it is his only real card because the concessions offered by Home Secretary Jacqui Smith at the meeting of Labour MPs last Monday are not given meaningful form in the amendments to the bill. It is surely significant that three of the most senior Labour lawyers - men who are used to divining the effect of legislation on the law - have not wavered. DPP Ken Macdonald, former Lord Chancellor Lord Falconer and former Attorney General Lord Goldsmith are all still vehemently opposed. None of them diminishes the terrorist threat either.

And if this is not enough, the Joint Committee on Human Rights came out with a report that said the amendments did not provide adequate protection against the threat of arbitrary detention. Andrew Dismore, the committee's Labour chairman, said: 'The government has talked of a major emergency... yet the amendments tabled by the government provide for possible events falling well short of that.'

Liberty, which has fought a long and effective campaign on this issue, pointed out that there is 'still no legal requirement for a terrorist emergency to exist and no requirement for the Home Secretary to show that 42-day detention is urgently needed to deal with the threat'. While Jacqui Smith may act in the spirit of her concessive presentation to Labour MPs, she will not be subject to legal restriction and nobody should forget that 42- day detention in Britain may be triggered by a threat anywhere in the world.

The amendments offer no hiding place for an MP with a serious political conscience or, for that matter, an interest in making good law. Because even on the other side of this debate there are doubters. The police and counterterrorist officials regard the amendments as unworkable because of concerns about disclosing operational details to the heads of three parliamentary committees in fast-moving situations.

This is not about Gordon Brown. However, reading his article in the Times last week, I was struck by the disturbing echo of Blair's 2002 WMD dossier. He may be sincere, but his conjuring of nightmares, the many hypotheticals followed by solemn avowals of principle and statesmanship, was exactly the formula which took us into Iraq. He is still talking in the language of the war on terror, a campaign that turned out to be as much against the rule of law as terrorism and which has caused the death of hundreds of thousands of people, enabled torture in Guantanamo and, as the Guardian revealed last week, the unlawful detention of suspects in nightmarish prison ships.

The proposal to hold people for six weeks without charge, or even giving them a reason, is part of that desperate, panicky convulsion which has seen the end of so many liberties in Britain.

Last week, John Major echoed the arguments, made here over the last two-and-half years in some 50 columns, that Britain's democracy and liberties can no longer be guaranteed. He said in the Times that 42 days was part and parcel 'of the total loss of privacy to an intrusive state with authoritarian tendencies. This is not a United Kingdom I recognise and Parliament should not accept it'.

That is absolutely right. In many respects, Sir John had a much tougher time than Gordon Brown, but at no stage did he suggest to his backbenchers that his and his party's survival could be won at the expense of habeus corpus, the idea born in Magna Carta which has inspired every democracy that ever existed.

This proposal is self-evidently wrong and everyone knows that. It is unnecessary because the police have not yet needed to hold a suspect for the current maximum of 28 days. On the evidence of recent plots, terrorist planning - if anything - seems to be getting cruder rather than more sophisticated. And finally the measure is counterproductive because it attacks the essence of what we are defending and so allows al-Qaeda to believe that we stand for nothing, that we will fold at the first whiff of trouble. This is not a Britain I recognise either.

I was very struck recently by a brief exchange I had with senior opposition politician who remarked on the negative atmosphere in the House of Commons, the lack of hope and generosity and the sheer unpleasantness of politics, a great contrast to the spirit surging through American politics at the moment. I have no doubt that this is a hangover from the Blair years, which were much more contemptuous of our institutions than most of us realised. Brown has not broken this pattern.

With each new attack on freedom and rights, this negative vibe becomes stronger and more clearly pathological. If the vote is won by Brown, Labour will be at odds with what the party knows to be right, which will be destructive for both us and them and bad for politics and our national life.

Jacqui Smith has blithely suggested that if it becomes a confidence vote, there would be massive support for Brown. 'I don't think it would be a problem,' she said. There speaks a former whip with a limited understanding of the sacred rights she intends to sacrifice for her party's sake. She might ask herself exactly what confidence will be expressed by victory - confidence in the Prime Minister's strength or his weakness?